EP0255005B1 - Process and apparatus for manufacturing metallic material by hot isostatic compacting of metal powder - Google Patents

Process and apparatus for manufacturing metallic material by hot isostatic compacting of metal powder Download PDF

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EP0255005B1
EP0255005B1 EP87110429A EP87110429A EP0255005B1 EP 0255005 B1 EP0255005 B1 EP 0255005B1 EP 87110429 A EP87110429 A EP 87110429A EP 87110429 A EP87110429 A EP 87110429A EP 0255005 B1 EP0255005 B1 EP 0255005B1
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Prior art keywords
pressure
reaction gas
metal powder
gas
capsule
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German (de)
French (fr)
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EP0255005A2 (en
EP0255005A3 (en
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Heinrich K. Prof. Dr.-Ing. Feichtinger
Markus O. Prof. Dr. Speidel
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Vereinigte Schmiedewerke GmbH
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Vereinigte Schmiedewerke GmbH
Schmiedewerke Krupp Kloeckner GmbH
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/12—Both compacting and sintering
    • B22F3/14—Both compacting and sintering simultaneously
    • B22F3/15—Hot isostatic pressing

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  • the invention relates to a method for producing metallic materials by hot isostatic pressing of metal powder or metal powder mixtures, collectively referred to as "metal powder", with high, in particular over 1%, contents of gases or vapors, in particular nitrogen, which are caused by pressurizing the metal powder with pressurized metal Reaction gas can be introduced into the material at a higher temperature.
  • the invention also includes an apparatus for performing this method.
  • Nitrogen is a substitution element and can partially replace other alloying elements, for example nickel. Nitrogen is an austenite stabilizing element.
  • nitride-forming elements such as chromium, vanadium, niobium, zirconium can also lead to the elimination of nitride phases, which have a favorable effect on the mechanical properties via the mechanism of precipitation hardening .
  • this effect can also have an effect on improving the creep resistance in the higher temperature range.
  • Highly embroidered steels can be produced using a melt metallurgical process. Steels whose nitrogen content is so low that bubbles do not form during solidification can also be melted with conventional melting units, such as the electric arc furnace or the induction furnace.
  • the nitrogen is added in the form of a nitrogen-containing ferro alloy, such as ferrochrome or ferromanganese.
  • ferrochrome or ferromanganese Such special ferro alloys are, however, very expensive. Therefore, this melt-metallurgical route for the production of embroidered steels is relatively expensive.
  • this remelting process creates a primary structure which, while having the advantages inherent in the electroslag remelting process, has a particularly high quality, but nevertheless still certain It has disadvantages with regard to inhomogeneities, as is common to all melt metallurgical processes with regard to micro and macro segregation, blowholes, pores and other defects.
  • the final material properties can possibly only be achieved by a thermomechanical aftertreatment, and the final shape, for example in the case of cap rings, can only be achieved by extensive machining.
  • Materials with high nitrogen contents can also be produced by powder metallurgy, for example by hot isostatic pressing.
  • This process is described for example in "Encyclopedia of Materials Science and Engineering", vol. 3, 1986, page 2186.
  • the metal powder to be pressed is filled into a deformable capsule, the air is sucked out of the capsule, the capsule and its contents are heated to an elevated temperature, and at the same time pressure is applied to all sides via a suitable medium, in particular gas exerted on the capsule and the metal powder contained therein.
  • a suitable medium in particular gas exerted on the capsule and the metal powder contained therein.
  • the powder is compacted into a compact body, with the elevated temperature causing a sintering process with solid-state diffusion, sometimes even in the presence of a liquid phase.
  • CH-PS 486 563 describes a process for producing a heat-resistant material in which a powder mixture consisting of a component A (iron, cobalt or nickel) with a component B (nitride or nitride mixture) and a component C (reactive tionable aluminum compound) is subjected to a pressure sintering process.
  • the powder components must be in a fine and homogeneous distribution. It is preferred if the individual components are present in a grain spectrum of 1-10 11 m.
  • the resulting metal-ceramic materials are hard, strong and resistant to chemicals, thermal shock and high temperature.
  • EP-OS 165 732 describes a process for the production of bodies consolidated with titanium nitride in dispersion, in which a powder consisting essentially of a titanium-containing ferritic or austenitic steel, preferably with 20% chromium and 25% nickel, first in a hydrogen-containing one Atmosphere is pre-sintered so that a body with sufficient strength is formed, which is, however, gas-permeable for the subsequent nitriding treatment. Then the pre-sintered body is flowed through with a hydrogen-nitrogen mixture in the temperature range of 1000-1150 ° C until the desired nitride content has been reached.
  • the bodies produced in this way can then either be subjected to a subsequent compression treatment by rolling or forging, or a nitrided powder can be produced by grinding, which can be processed further using the known methods of powder metallurgy.
  • a metal powder produced by atomizing a melt is filled into a steel container and successively degassed and nitrided.
  • the powder is exposed to an unspecified nitrogen pressure at 1150 ° C for two hours.
  • the resulting nitrogen content in the powder is a function of the treatment temperature, the nitrogen pressure, the nitration time and the mass transfer coefficient.
  • Such an embroidered powder was then hot isostatically pressed at 2000 bar and 1100 ° C. for two hours.
  • US Pat. No. 4,140,527 provides for the nitrogen powder to flow through the metal powder for a longer period of time, for example by embroidering at a higher temperature, but the final nitrogen content is and is the result of a complex interplay of pressure, temperature and time therefore difficult to set precisely.
  • the nitriding of the powder is a separate process step, associated with considerable effort, the nitriding time being very long, in particular because of the low nitrogen pressure provided.
  • the maximum achievable nitrogen content in equilibrium is limited to values of only 1% at most.
  • the invention has for its object to provide a method for hot isostatic pressing of metal powders, in particular steel powders, with which workpieces with high gas contents up to over 1% can be produced in a simple manner.
  • reaction gas diffuses from the pores into the body formed by the compacting and is stored in it either in a solid solution or in the form of precipitates, in particular metal nitrides or carbonitrides.
  • the process is expediently controlled in such a way that a continuous process occurs, i.e. that the pressures and temperature are continuously increased until the maximum values are reached.
  • the process gas is a mixture of a gas which is insoluble in powder and the intended reaction gas, the volume fraction of which is selected such that it is approximately in equilibrium with the gas content of the metal phase under the conditions prevailing during the sintering phase. It can then be achieved that diffusion cannot lead to different gas concentrations in the edge region of the compact. In the absence of equilibrium, diffusion would result through the capsule, which would either lead to depletion or to an accumulation of reaction gas at least in the edge region of the compact.
  • the conditions (temperature and pressure) during hot isostatic pressing and the composition of the metal powder can be selected so that the gas introduced into the material remains in solid solution.
  • solid solution hardening in a manner known per se both the mechanical properties, such as strength and toughness, and the chemical properties, such as stress corrosion resistance, are influenced in a favorable manner.
  • the metal powder can also be alloyed with elements which form compounds (nitrides) with the reaction gas, in particular nitrogen, which are then present in the material as finely divided precipitates.
  • the reaction gas in particular nitrogen
  • the high-temperature stability of steel alloys is brought about in particular by the fine precipitation of special nitrides.
  • a mixture of argon and nitrogen is particularly suitable as the process gas.
  • the device for carrying out the claimed method is characterized by a pressure vessel with a heating element (s) and a process gas supply line and a capsule in the pressure chamber, filled with metal powder, to which a reaction gas supply line is connected.
  • a fine-grained powder which corresponds in its composition to the corrosion-resistant and non-magnetizable cap ring steel X10CrMn1818, is filled into a steel capsule suitable for the subsequent sintering process under the usual precautionary measures, such as protection against oxidation, whereby it may then be subjected to a vacuum treatment.
  • the metal powder used has an intermediate grain volume of 30% by volume, depending on its bulk density.
  • the capsule 2 filled with the powder mixture 1 is introduced into a pressure vessel 3 with a heating furnace 4 for hot isostatic pressing.
  • the capsule 2 is connected to a container (not shown) by means of a line 6 leading through the outer wall in order to set the desired nitrogen pressure (reaction gas) in the interior of the capsule.
  • the process gas originating from a further reservoir (also not shown) and necessary for the compression process can be admitted into the autoclave via a further line 5.
  • the process gas consists of argon with an admixture of 3% by volume nitrogen.
  • the gas supply to capsule 2 is interrupted and the process gas mixture, consisting of 3% by volume nitrogen and 97% by volume argon, is now increased to a final pressure of 2000 bar.
  • the temperature is increased to 1250 ° C at the same time.
  • the concentration of 3% by volume in the process gas is in equilibrium with the 3% nitrogen in the powder.
  • the capsule 2 used in the above process can already largely have the shape of a ring which is similar to the shape of the cap ring to be produced, so that a minimum of machining post-processing steps is created.

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  • Manufacturing & Machinery (AREA)
  • Mechanical Engineering (AREA)
  • Powder Metallurgy (AREA)

Abstract

1. A process for producing metallic materials by the isostatic pressing of metal powders with high contents of gases which are introduced into the metal powder by acting on the metal powder with pressurized reaction gas at elevated temperature, characterized in that : a) after being filled in known manner into a capsule the metal powder is acted upon with the reaction gas, while b) at the same time a process acts on the capsule with the supply of heat from outside under pressure, and c) at the same time as the pressure of the process gas is increased the pressure of the reaction gas is increased approximately identically, d) the pressure being increased until a quantity of reaction gas is enclosed in the volume between the grains of metal powder which is adequate to achieve the required alloying content under the temperature conditions prevailing therein, and then e) the further supply of reaction gas is terminated and f) the pressure of the process gas outside the capsule and the temperature are brought to a value necessary for the isostatic hot pressing.

Description

Die Erfindung betrifft ein Verfahren zur Herstellung metallischer Werkstoffe durch isostatisches Heißpressen von Metallpulver oder Metallpulvergemischen, zusammenfassend als "Metallpulver" bezeichnet, mit hohen, insbesondere über 1% liegenden Gehalten an Gasen oder Dämpfen, insbesondere Stickstoff, die durch Beaufschlagen des Metallpulvers mit unter Druck stehendem Reaktionsgas bei höherer Temperatur in den Werkstoff eingebracht werden. Die Erfindung umfaßt gleichfalls eine Vorrichtung zur Durchführung dieses Verfahrens.The invention relates to a method for producing metallic materials by hot isostatic pressing of metal powder or metal powder mixtures, collectively referred to as "metal powder", with high, in particular over 1%, contents of gases or vapors, in particular nitrogen, which are caused by pressurizing the metal powder with pressurized metal Reaction gas can be introduced into the material at a higher temperature. The invention also includes an apparatus for performing this method.

Es ist bekannt, daß zum Beispiel das Einbringen großer Stickstoffmengen in Stahllegierungen in manchen Fällen zu einer entscheidenden Verbesserung der Werkstoffeigenschaften führen kann. Dabei kann dieser Stickstoffgehalt in manchen Fällen auf Zwischengitterplätzen in Lösung vorliegen, wobei man durch Zulegieren von Elementen, wie Chrom oder Mangan, welche die Stickstofflöslichkeit erhöhen, größere Mengen an Stickstoff in die Matrix einbringen kann. Die günstige Wirkung des Stickstoffs erfolgt in solchen Fällen über den Mechanismus der Mischkristallhärtung, ähnlich wie dies auch durch Kohlenstoff bewirkt wird. Dabei muß keine Abnahme der Zähigkeit in Kauf genommen werden. Ein Beispiel eines solchen Werkstoffs ist ein Stahl mit 18% Chrom und 18% Mangan, der bei Stickstoffgehalten bis zu 1 % zu einem Werkstoff führt, welcher infolge seiner kombinierten Festigkeits- und Zähigkeitseigenschaften ideal für die Herstellung von Kappenringen ist. (Alle vorstehend und nachfolgend gemachten %-Angaben sind, sofern nichts anders angegeben ist, Gewichtsprozent).It is known that, for example, the introduction of large amounts of nitrogen into steel alloys can lead to a decisive improvement in the material properties in some cases. In some cases this nitrogen content can be present in solution at interstitial sites, it being possible to introduce larger amounts of nitrogen into the matrix by adding elements such as chromium or manganese which increase the nitrogen solubility. In such cases, the beneficial effect of nitrogen is achieved through the mechanism of mixed crystal hardening, similar to the way it is caused by carbon. There is no need to accept a decrease in toughness. An example of such a material is a steel with 18% chromium and 18% manganese, which with nitrogen contents up to 1% leads to a material which is ideal for the production of cap rings due to its combined strength and toughness properties. (All percentages given above and below are, unless stated otherwise, percentages by weight).

Neben der Verbesserung der mechanischen Eigenschaften zeigt sich in vielen Fällen auch eine günstige Beeinflussung des Korrosionsverhaltens, insbesondere der Beständigkeit gegen Spannungsrißkorrosion.In addition to the improvement of the mechanical properties, in many cases there is also a favorable influence on the corrosion behavior, in particular the resistance to stress corrosion cracking.

Stickstoff ist ein Substitutionselement und kann andere Legierungselemente, zum Beispiel Nickel, teilweise ersetzen. Stickstoff ist ein Austenit-stabilisierendes Element.Nitrogen is a substitution element and can partially replace other alloying elements, for example nickel. Nitrogen is an austenite stabilizing element.

Neben der Lösung von Stickstoff auf Zwischengitterplätzen kann es bei Vorliegen einer bestimmten gelösten Stickstoffmenge im Verein mit Nitrid-bildenden Elementen, wie Chrom, Vanadium, Niob, Zirkonium, auch zur Ausscheidung von Nitridphasen kommen, welche die mechanischen Eigenschaften über den Mechanismus der Ausscheidungshärtung günstig beeinflussen. Dabei kann sich diese Wirkung beim Vorliegen hochtemperaturstabiler Nitride auch im höheren Temperturbereich auf eine Verbesserung des Kriechwiderstands auswirken.In addition to the solution of nitrogen in interstitial sites, a certain amount of dissolved nitrogen in combination with nitride-forming elements such as chromium, vanadium, niobium, zirconium can also lead to the elimination of nitride phases, which have a favorable effect on the mechanical properties via the mechanism of precipitation hardening . In the presence of high-temperature stable nitrides, this effect can also have an effect on improving the creep resistance in the higher temperature range.

Die Herstellung hochgestickter Stähle kann auf schmelzmetallurgischem Weg erfolgen. Dabei können Stähle, deren Stickstoffgehalt so niedrig ist, daß es während der Erstarrung nicht zur Blasenbildung kommt, auch mit konventionellen Schmelzaggregaten, wie dem Elektrolichtbogenofen oder dem Induktionsofen, erschmolzen werden. Der Stickstoff wird in Form einer stickstoffhaltigen Ferrolegierung, wie Ferrochrom oder Ferromangan, zugegeben. Derartige spezielle Ferrolegierungen sind allerdings sehr teuer. Daher ist dieser schmelzmetallurgische Weg zur Herstellung aufgestickter Stähle verhältnismäßig kostenaufwendig.Highly embroidered steels can be produced using a melt metallurgical process. Steels whose nitrogen content is so low that bubbles do not form during solidification can also be melted with conventional melting units, such as the electric arc furnace or the induction furnace. The nitrogen is added in the form of a nitrogen-containing ferro alloy, such as ferrochrome or ferromanganese. Such special ferro alloys are, however, very expensive. Therefore, this melt-metallurgical route for the production of embroidered steels is relatively expensive.

Höhere Stickstoffgehalte lassen sich nur in die Schmelze einbringen, wenn über dem Metallschmelzbad ein hoher Gasdruck aufrechterhalten wird. So werden zum Beispiel in einer Druck-Elektroschlackenumschmelzanlage Stahlblöcke mit einem Gewicht von 14 t unter einem Gasdruck von bis 42 bar umgeschmolzen. Dabei geht man meist von einer Umschlmezelektrode aus, die schon einen möglichst hohen Stickstoffgehalt besitzt. Die weitere Aufstickung erfolgt durch laufende Zugabe einer entsprechenden Metalinitridmenge, zum Beispiel Siliziumnitrid, in die Schlacke. Nach Zersetzung des Nitrids kommt es zu einer Aufnahme des Stickstoffs durch das Metallbad.Higher nitrogen contents can only be introduced into the melt if a high gas pressure is maintained over the molten metal bath. For example, in a pressure electro-slag remelting system, steel blocks weighing 14 t are remelted under a gas pressure of up to 42 bar. This is usually based on a coating electrode that already has the highest possible nitrogen content. Further nitriding takes place by continuously adding a corresponding amount of metal nitride, for example silicon nitride, to the slag. After the nitride has decomposed, the nitrogen is absorbed by the metal bath.

Neben dem verfahrenstechnischen Aufwand, der mit der Erzeugung einer geeigneten Elektrode für den Elektroschlackenumschmelzprozeß und mit der genau kontrollierten Druckerzeugung verbunden ist, entsteht bei diesem Umschmelzprozeß ein Primärgefüge, das entsprechend den dem Elektroschlackenumschmelzprozeß immanenten Vorteilen zwar eine besonders hohe Qualität hat, das jedoch trotzdem noch gewisse Nachteile in bezug auf Inhomogenitäten hat, wie dies allen schmelzmetallurgischen Prozessen in bezug auf Mikro- und Makroseigerungen, auf Lunker, Poren und andere Fehler eigen ist. Darüber hinaus können die endgültigen Werkstoffeigenschaften eventuell nur durch eine thermomechanische Nachbehandlung erzielt werden, und die endgültige Form ist zum Beispiel bei Kappenringen nur durch eine umfangreiche spanende Bearbeitung erzielbar.In addition to the process engineering effort associated with the production of a suitable electrode for the electroslag remelting process and with the precisely controlled pressure generation, this remelting process creates a primary structure which, while having the advantages inherent in the electroslag remelting process, has a particularly high quality, but nevertheless still certain It has disadvantages with regard to inhomogeneities, as is common to all melt metallurgical processes with regard to micro and macro segregation, blowholes, pores and other defects. In addition, the final material properties can possibly only be achieved by a thermomechanical aftertreatment, and the final shape, for example in the case of cap rings, can only be achieved by extensive machining.

Werkstoffe mit hohen Stickstoffgehalten können auch auf pulvermetallurgischem Weg, zum Beispiel durch isostatisches Heißpressen, erzeugt werden. Dieses Verfahren ist beispielsweise beschrieben in "Encyclopedia of Materials Science and Engineering", vol. 3, 1986, Seite 2186. Das zu pressende Metallpulver wird in eine verformbare Kapsel eingefüllt, die Luft wird aus der Kapsel abgesaugt, die Kapsel mit Inhalt wird auf erhöhte Temperatur erwärmt, und gleichzeitig wird über ein geeignetes Medium, insbesondere Gas, allseits ein Druck auf die Kapsel und das in ihr befindliche Metallpulver ausgeübt. Bei diesem isostatischen Heißpressen erfolgt eine Verdichtung des Pulvers zu einem kompakten Körper, wobei durch die erhöhte Temperatur ein Sintervorgang mit Festkörperdiffusion, zum Teil auch in Anwesenheit flüssiger Phase, stattfindet.Materials with high nitrogen contents can also be produced by powder metallurgy, for example by hot isostatic pressing. This process is described for example in "Encyclopedia of Materials Science and Engineering", vol. 3, 1986, page 2186. The metal powder to be pressed is filled into a deformable capsule, the air is sucked out of the capsule, the capsule and its contents are heated to an elevated temperature, and at the same time pressure is applied to all sides via a suitable medium, in particular gas exerted on the capsule and the metal powder contained therein. In this isostatic hot pressing, the powder is compacted into a compact body, with the elevated temperature causing a sintering process with solid-state diffusion, sometimes even in the presence of a liquid phase.

In der CH-PS 486 563 ist ein Verfahren zur Herstellung eines hitzebeständigen Werkstoffs beschrieben, bei dem ein Pulvergemisch, bestehend aus einer Komponente A (Eisen, Kobalt oder Nickel) mit einer Komponente B (Nitrid oder Nitridgemisch) sowie einer Komponente C (reaktionsfähige Aluminiumverbindung) einem Drucksinterprozeß ausgesetzt wird. Die Pulverkomponenten müssen dabei in feiner und homogener Verteilung vorliegen. Es wird bevorzugt, wenn die einzelnen Komponenten in einem Kornspektrum von 1-10 11m vorliegen. Die entstehenden Metallkeramik-Werkstoffe sind hart, fest und beständig gegen Chemikalien, thermoschockbeständig und hochtemperaturbeständig.CH-PS 486 563 describes a process for producing a heat-resistant material in which a powder mixture consisting of a component A (iron, cobalt or nickel) with a component B (nitride or nitride mixture) and a component C (reactive tionable aluminum compound) is subjected to a pressure sintering process. The powder components must be in a fine and homogeneous distribution. It is preferred if the individual components are present in a grain spectrum of 1-10 11 m. The resulting metal-ceramic materials are hard, strong and resistant to chemicals, thermal shock and high temperature.

In der EP-OS 165 732 wird ein Verfahren zur Herstellung von mit Titannitrid dispersionsverfestigten Körpern beschrieben, bei dem ein im wesentlichen aus einem Titan-haltigen ferritischen oder austenitischen Stahl bestehendes Pulver, bevorzugt mit 20% Chrom und 25% Nickel, zuerst in einer wasserstoffhaltigen Atmosphäre vorgesintert wird, so daß ein Körper mit ausreichender Festigkeit entsteht, der jedoch für die nachfolgende Nitrierbehandlung gasdurchlässig ist. Anschließend wird der vorgesinterte Körper mit einem Wasserstoff-Stickstoff-Gemisch im Temperaturbereich von 1000-1150°C durchströmt, bis sich der gewünschte Nitridgehalt eingestellt hat. Die so hergestellten Körper können anschließend entweder einer nachfolgenden Verdichtungsbehandlung durch Walzen oder Schmieden unterworfen werden, oder es kann durch Mahlen ein nitriertes Pulver hergestellt werden, welches mit den bekannten Methoden der Pulvermetallurgie weiterverarbeitbar ist. Die US-PS 4 140 527 beschreibt die pulvermetallurgische Herstellung von stickstoffhaltigen Werkzeugstählen mit folgender Zusammensetzung: mindestens 0,4% Stickstoff, 1,6-15% Vanadium, Kohlenstoff+Stickstoff=1/5 des Gehalts an Vanadium einschließlich einer zusätzlichen Menge von mindestens 0,2-0,5% sowie wahlweise bis 15% Chrom, bis 10% Molybdän, bis 20% Wolfram und bis 15% Kobalt einzeln oder zu mehreren. Gemäß einem Ausführungsbeispiel wird ein durch Zerstäuben einer Schmelze erzeugtes Metallpulver in einen Stahlbehälter abgefüllt und aufeinanderfolgend entgast und nitriert. Dabei wird das Pulver bei 1150°C während zwei Stunden einem nicht näher bezeichneten Stickstoffdruck ausgesetzt. Der im Pulver resultierende Stickstoffgehalt ergibt sich als Funktion der Behandlungstemperatur, des Stickstoffdrucks, der Nitrierdauer und des Stoffübergangskoeffizienten. Ein derart aufgesticktes Pulver wurde anschließend bei 2000 bar und 1100°C während zwei Stunden isostatisch heißgepreßt.EP-OS 165 732 describes a process for the production of bodies consolidated with titanium nitride in dispersion, in which a powder consisting essentially of a titanium-containing ferritic or austenitic steel, preferably with 20% chromium and 25% nickel, first in a hydrogen-containing one Atmosphere is pre-sintered so that a body with sufficient strength is formed, which is, however, gas-permeable for the subsequent nitriding treatment. Then the pre-sintered body is flowed through with a hydrogen-nitrogen mixture in the temperature range of 1000-1150 ° C until the desired nitride content has been reached. The bodies produced in this way can then either be subjected to a subsequent compression treatment by rolling or forging, or a nitrided powder can be produced by grinding, which can be processed further using the known methods of powder metallurgy. US Pat. No. 4,140,527 describes the powder metallurgical production of nitrogen-containing tool steels with the following composition: at least 0.4% nitrogen, 1.6-15% vanadium, carbon + nitrogen = 1/5 of the vanadium content including an additional amount of at least 0.2-0.5% and optionally up to 15% chromium, up to 10% molybdenum, up to 20% tungsten and up to 15% cobalt individually or in pairs. According to one embodiment, a metal powder produced by atomizing a melt is filled into a steel container and successively degassed and nitrided. The powder is exposed to an unspecified nitrogen pressure at 1150 ° C for two hours. The resulting nitrogen content in the powder is a function of the treatment temperature, the nitrogen pressure, the nitration time and the mass transfer coefficient. Such an embroidered powder was then hot isostatically pressed at 2000 bar and 1100 ° C. for two hours.

Die US-PS 4 140 527 sieht zwar vor, das Metallpulver zum Beispiel durch Aufsticken bei höherer Temperatur über einen längeren Zeitraum von einer stickstoffhaltigen Atmosphäre durchströmen zu lassen, jedoch ist der endgültige Stickstoffgehalt das Ergebnis eines komplexen Zusammenspiels von Druck, Temperatur und Zeit und ist daher nur mit Schwierigkeiten genau einzustellen. Darüber hinaus ist die Aufstickung des Pulvers ein gesonderter Verfahrensschritt, verbunden mit erheblichem Aufwand, wobei insbesondere wegen des vorgesehenen niedrigen Stickstoffdrucks die Aufstickungszeit sehr lang ist. Insbesondere ist aber wegen des niedrigen Stickstoffgesamtdrucks der im Gleichgewicht maximal erzielbare Stickstoffgehalt auf Werte von nur maximal 1 % beschränkt.US Pat. No. 4,140,527 provides for the nitrogen powder to flow through the metal powder for a longer period of time, for example by embroidering at a higher temperature, but the final nitrogen content is and is the result of a complex interplay of pressure, temperature and time therefore difficult to set precisely. In addition, the nitriding of the powder is a separate process step, associated with considerable effort, the nitriding time being very long, in particular because of the low nitrogen pressure provided. In particular, however, due to the low total nitrogen pressure, the maximum achievable nitrogen content in equilibrium is limited to values of only 1% at most.

Ausgehend von diesem Stand der Technik liegt der Erfindung die Aufgabe zugrunde, ein Verfahren zum isostatischen Heißpressen von Metallpulvern, insbesondere Stahlpulvern zu schaffen, mit dem in einfacher Weise Werkstücke mit hohen Gasgehalten bis über 1% herstellbar sind.Based on this prior art, the invention has for its object to provide a method for hot isostatic pressing of metal powders, in particular steel powders, with which workpieces with high gas contents up to over 1% can be produced in a simple manner.

Gelöst wird diese Aufgabe durch die im Anspruch 1 angegebenen Maßnahmen. Der wesentliche Unterschied zwischen dem erfindungsgemäßen Verfahren und dem aus der US-PS 4 140 527 bekannten besteht darin, daß der Druck des Reaktionsgases und des Prozeßgases gleichermaßen gesteigert wird. Dadurch gelingt es, einen sehr hohen Reaktionsgasdruck aufzubauen als Voraussetzung für das Einbringen hoher Gasgehalte in das Metallpulver. Ohne entsprechende Erhöhung des Prozeßgasdrucks als Gegendruck wären solche Reaktionsgas- drücke nicht erzielbar, die Kapsel würde vorher gesprengt.This problem is solved by the measures specified in claim 1. The essential difference between the method according to the invention and that known from US Pat. No. 4,140,527 is that the pressure of the reaction gas and the process gas is increased equally. This makes it possible to build up a very high reaction gas pressure as a prerequisite for introducing high gas contents into the metal powder. Without a corresponding increase in the process gas pressure as a counter pressure, such reaction gas pressures would not be achievable, the capsule would be blown up beforehand.

Nach dem Einbringen einer dem Gasdruck entsprechenden hohen Reaktionsgasmenge in das Zwischenkornvolumen des Metallpulvers und dem Abschalten der weiteren Zufuhr von Reaktionsgas in die Kapsel werden der Druck des Prozeßgases und die Temperatur so weit gesteigert, bis die Bedingungen für das isostatische Heißpressen eingestellt sind. Bei diesen Bedingungen diffundiert das Reaktionsgas aus den Poren in den sich durch die Kompaktierung bildenden Körper und lagert sich in ihm entweder in fester Lösung oder in Form von Ausscheidungen insbesondere Metallnitriden oder -carbonitriden, ein.After introducing a large amount of reaction gas corresponding to the gas pressure into the intermediate grain volume of the metal powder and switching off the further supply of reaction gas into the capsule, the pressure of the process gas and the temperature are increased until the conditions for hot isostatic pressing are set. Under these conditions, the reaction gas diffuses from the pores into the body formed by the compacting and is stored in it either in a solid solution or in the form of precipitates, in particular metal nitrides or carbonitrides.

Zweckmäßigerweise wird der Prozeß so gesteuert, daß sich ein kontinuierlicher Ablauf einstellt, d.h. daß Drücke und Tempertur fortlaufend bis zum Erreichen der Höchstwerte gesteigert werden.The process is expediently controlled in such a way that a continuous process occurs, i.e. that the pressures and temperature are continuously increased until the maximum values are reached.

Gemäß einer bevorzugten Ausgestaltung des erfindungsgemäßen Verfahrens ist das Prozeßgas ein Gemisch aus einem in Pulver nicht löslichen Gas und dem vorgesehenen Reaktionsgas, dessen Volumenanteil so gewählt wird, daß es unter den während der Sinterphase herrschenden Bedingungen annähernd im Gleichgewicht steht zu dem Gasgehalt der Metallphase. Es kann dann nämlich erreicht werden, daß es im Randbereich des Preßlings durch Diffusion nicht zu unterschiedlichen Gaskonzentrationen kommen kann. Bei fehlendem Gleichgewicht würde sich nämlich durch die Kapsel hindurch eine Diffusion ergeben, die entweder zu einer Verarmung oder zu einer Anreicherung an Reaktionsgas mindestens im Randbereich des Preßlings führen würde.According to a preferred embodiment of the process according to the invention, the process gas is a mixture of a gas which is insoluble in powder and the intended reaction gas, the volume fraction of which is selected such that it is approximately in equilibrium with the gas content of the metal phase under the conditions prevailing during the sintering phase. It can then be achieved that diffusion cannot lead to different gas concentrations in the edge region of the compact. In the absence of equilibrium, diffusion would result through the capsule, which would either lead to depletion or to an accumulation of reaction gas at least in the edge region of the compact.

Die Bedingungen (Temperatur und Druck) beim isostatischen Heißpressen und die Zusammensetzung des Metallpulvers können so gewählt werden, daß das in den Werkstoff eingebrachte Gas in fester Lösung bleibt. Durch Mischkristallhärtung werden in an sich bekannter Weise dann sowohl die mechanischen Eigenschaften, wie Festigkeit und Zähigkeit, als auch die chemischen Eigenschaften, wie Spannungsrißkorrosionsbeständigkeit, in günstiger Weise beeinflußt.The conditions (temperature and pressure) during hot isostatic pressing and the composition of the metal powder can be selected so that the gas introduced into the material remains in solid solution. By means of solid solution hardening in a manner known per se both the mechanical properties, such as strength and toughness, and the chemical properties, such as stress corrosion resistance, are influenced in a favorable manner.

Das Metallpulver kann jedoch auch mit Elementen legiert sein, die mit dem Reaktionsgas, insbesondere Stickstoff, Verbindungen (Nitride) eingehen, die dann im Werkstoff als fein verteilte Ausscheidungen vorliegen. Durch feinteilige Ausscheidung von Sondernitriden wird insbesondere die Hochtemperaturstabilität von Stahllegierungen bewirkt.However, the metal powder can also be alloyed with elements which form compounds (nitrides) with the reaction gas, in particular nitrogen, which are then present in the material as finely divided precipitates. The high-temperature stability of steel alloys is brought about in particular by the fine precipitation of special nitrides.

Als Prozeßgas eignet sich vor allem ein Gemisch aus Argon und Stickstoff.A mixture of argon and nitrogen is particularly suitable as the process gas.

Die Vorrichtung zur Durchführung des beanspruchten Verfahrens ist gekennzeichnet durch ein Druckgefäß mit Heizelement(en) und einer Prozeßgas-Zuleitung sowie einer im Druckraum befindlichen, mit Metallpulver gefüllten Kapsel, an die eine Reaktionsgas-Zuleitung angeschlossen ist.The device for carrying out the claimed method is characterized by a pressure vessel with a heating element (s) and a process gas supply line and a capsule in the pressure chamber, filled with metal powder, to which a reaction gas supply line is connected.

Die Vorteile des erfindungsgemäßen Verfahrens können in folgender Weise zusammengefaßt werden:

  • Grundsätzliche Vorteile des pulvermetallurgischen Weges gegenüber schmelzmetallurgischen Verfahren, d.h. Gleichmäßigkeit von Zusammensetzung und Gefüge, was eine Gleichmäßigkeit der mechanischen Eigenschaften in verschiedenen Beanspruchungsrichtungen sowie an verschiedenen Stellen des hergestellten Werkstoffs ergibt, werden übernommen,
  • genaue Dosierbarkeit des Stickstoffgehaltes, welcher nicht über eine thermodynamische Beziehung eingemessen wird, sondern das Resultat einer exakt durchführbaren Druck- und Temperaturmessung ist,
The advantages of the method according to the invention can be summarized in the following way:
  • Fundamental advantages of the powder metallurgical method compared to melt metallurgical processes, ie uniformity of composition and structure, which results in uniformity of the mechanical properties in different directions of stress as well as at different points in the material produced, are adopted,
  • precise dosing of the nitrogen content, which is not measured via a thermodynamic relationship, but is the result of an exactly feasible pressure and temperature measurement,

Möglichkeit der Erzielung von höchsten Gasgehalten, welche mit den heutigen Methoden der Schmelzmetallurgie gar nicht herstellbar sind,Possibility of achieving the highest gas contents, which cannot be produced with today's methods of melt metallurgy,

Möglichkeit der Herstellung von Halbzeug, welches weitgehend schon der Endform des zu erzielenden Werkstoffkörpers entspricht,Possibility of producing semi-finished products, which largely correspond to the final shape of the material body to be achieved,

Ersatz der teuren Stickstofflegierungen, z.B. in Form von hochaufgestickten Ferrolegierungen und Metallnitriden durch verhältnismäßig billiges Stickstoffgas.Replacement of the expensive nitrogen alloys, e.g. in the form of highly embroidered ferro alloys and metal nitrides using relatively cheap nitrogen gas.

Anhand eines Beispiels wird die Erfindung näher erläutert.The invention is explained in more detail using an example.

Beispielexample

Ein feinkörniges Pulver, welches in seiner Zusammensetzung dem korrosionsbeständigen und nichtmagnetisierbaren Kappenringstahl X10CrMn1818 entspricht, wird unter den üblichen Vorsichtsmaßnahmen, wie Schutz vor Oxidation etc., in eine für den nachfolgenden Sinterprozeß geeignete Stahlkapsel eingefüllt, wobei es anschließend eventuell noch einer Vakuumbehandlung unterworfen wird. Das verwendete Metallpulver besitzt entsprechend seiner Schüttdichte ein Zwischenkornvolumen von 30 Vol-%.A fine-grained powder, which corresponds in its composition to the corrosion-resistant and non-magnetizable cap ring steel X10CrMn1818, is filled into a steel capsule suitable for the subsequent sintering process under the usual precautionary measures, such as protection against oxidation, whereby it may then be subjected to a vacuum treatment. The metal powder used has an intermediate grain volume of 30% by volume, depending on its bulk density.

Entsprechend Fig. 1 wird die mit dem Pulvergemisch 1 gefüllte Kapsel 2 in einen Druckbehälter 3 mit Heizofen 4 zum isostatischen Heißpressen eingebracht. Dabei ist die Kapsel 2 mittels einer durch die Außenwandung führenden Leitung 6 mit einem nicht dargestellten Behälter verbunden, um im Inneren der Kapsel den gewünschten Stickstoffdruck (Reaktionsgas) einzustellen. Über eine weitere Leitung 5 kann das aus einem weiteren, ebenfalls nicht dargestellten Reservoir stammende und für den Verdichtungsprozeß nötige Prozeßgas in den Autoklaven eingelassen werden. Im vorliegenden Fall besteht das Prozeßgas aus Argon mit einer Beimischung von 3 Vol-% Stickstoff.1, the capsule 2 filled with the powder mixture 1 is introduced into a pressure vessel 3 with a heating furnace 4 for hot isostatic pressing. The capsule 2 is connected to a container (not shown) by means of a line 6 leading through the outer wall in order to set the desired nitrogen pressure (reaction gas) in the interior of the capsule. The process gas originating from a further reservoir (also not shown) and necessary for the compression process can be admitted into the autoclave via a further line 5. In the present case, the process gas consists of argon with an admixture of 3% by volume nitrogen.

In einer ersten Phase wird nun der Druck des Reaktionsgases in Form von in der Kapsel 2 simultan mit dem Druck des Argon-Stickstoff-Gasgemisches (Prozeßgas) im Autoklav erhöht, so daß die dünnwandige Kapsel 2 praktisch keinerlei Krafteinwirkungen erleidet. In Funktion der in Kapsel 2 herrschenden Temperatur und unter Annahme der ungefähren Gültigkeit des idealen Gasgesetzes wird der Druck nun solange erhöht, bis die im Zwischenvolumen befindliche gasförmige Stickstoffmenge der im endgültigen Werkstoff angestrebten Stickstoffmenge entspricht, wobei dieser Stickstoffdruck in folgender Weise errechnet werden kann:

Figure imgb0001
Dabei bedeuten:

  • p=Stickstoff-Fülldruck (N/m2)
  • %N=gewünschter Stickstoffgehalt in m-%
  • %Por=Porositätsanteil der Pulverschüttung
  • dpu=Dichte des Metallpulvers (kg/m3)
  • p°=Standarddruck (N/m 2)
  • T o=Standardtemperatur (K)
  • T=Temperatur beim Einfüllen (K)
  • V"",1=Molvolumen (m3)
  • AN=Atomgewicht von Stickstoff (kg)
In a first phase, the pressure of the reaction gas in the form of in the capsule 2 is increased simultaneously with the pressure of the argon-nitrogen gas mixture (process gas) in the autoclave, so that the thin-walled capsule 2 suffers practically no force. As a function of the temperature prevailing in capsule 2 and assuming the approximate validity of the ideal gas law, the pressure is now increased until the amount of gaseous nitrogen in the intermediate volume corresponds to the amount of nitrogen aimed for in the final material, which nitrogen pressure can be calculated in the following way:
Figure imgb0001
 Mean:
  • p = nitrogen filling pressure (N / m 2 )
  • % N = desired nitrogen content in m-%
  • % Por = proportion of porosity in the powder bed
  • dp u = density of the metal powder (kg / m 3 )
  • p ° = standard pressure (N / m 2 )
  • T o = standard temperature (K)
  • T = filling temperature (K)
  • V "", 1 = molar volume (m 3 )
  • A N = atomic weight of nitrogen (kg)

In der vorliegenden Legierung ist bei Raumtemperatur und dem Zwischenkornanteil von 30 Vol- % zur Erzielung eines Endstickstoffgehaltes von 3% ein Stickstoffdruck von ca. 465 bar nötig.In the present alloy, a nitrogen pressure of approx. 465 bar is necessary at room temperature and an intermediate grain content of 30% by volume to achieve a final nitrogen content of 3%.

Nach Abschluß des Füllprozesses wird die Gaszuleitung zur Kapsel 2 unterbrochen, und das Prozeßgasgemisch, bestehend aus 3 Vol-% Stickstoff und 97 Vol-% Argon wird nun auf einen Enddruck von 2000 bar erhöht. Dabei wird die Temperatur gleichzeitig auf 1250°C gesteigert. Unter diesen Bedingungen liegt die Konzentration von 3 Vol-% im Prozeßgas im Gleichgewicht mit den 3% Stickstoff im Pulver. Infolgedessen kommt es zu keiner Veränderung der Randkonzentration in bezug auf den Stickstoffgehalt. Diese Sinterbedingungen werden während 2 h beibehalten, anschließend wird ein Abkühlungsprogramm, gefolgtvon einem Druckentspannungsprogramm, durchlaufen.After the filling process has been completed, the gas supply to capsule 2 is interrupted and the process gas mixture, consisting of 3% by volume nitrogen and 97% by volume argon, is now increased to a final pressure of 2000 bar. The temperature is increased to 1250 ° C at the same time. Under these conditions the concentration of 3% by volume in the process gas is in equilibrium with the 3% nitrogen in the powder. As a result, there is no change in the marginal concentration with respect to the nitrogen content. These sintering conditions are maintained for 2 hours, followed by a cooling program followed by a pressure release program.

Die beim obigen Prozeß eingesetzte Kapsel 2 kann dabei schon weitgehend die Form eines Ringes haben, welcher der Form des herzustellenden Kappenringes ähnlich ist, so daß ein Minimum an spanabhebenden Nachbearbeitungsgängen entsteht.The capsule 2 used in the above process can already largely have the shape of a ring which is similar to the shape of the cap ring to be produced, so that a minimum of machining post-processing steps is created.

Claims (6)

1. A process for producing metallic materials by the isostatic pressing of metal powders with high contents of gases which are introduced into the metal powder by acting on the metal powder with pressurized reaction gas at elevated temperature, characterized in that:
a) after being filled in known manner into a capsule the metal powder is acted upon with the reaction gas, while
b) at the same time a process acts on the capsule with the supply of heat from outside under pressure, and
c) at the same time as the pressure of the process gas is increased the pressure of the reaction gas is increased approximately identically,
d) the pressure being increased until a quantity of reaction gas is enclosed in the volume between the grains of metal powder which is adequate to achieve the required alloying content under the temperature conditions prevailing therein, and then
e) the further supply of reaction gas is terminated and
f) the pressure of the process gas outside the capsule and the temperature are brought to a value necessary for the isostatic hot pressing.
2. A process according to Claim 1, characterized in that the process gas is a mixture of a gas insoluble in the metal powder and the provided reaction gas, the volumetric proportion of the reaction gas in the mixture so selected that under the pressure and temperature prevailing during the hot isostatic sintering phase it is approximately in equilibrium with the gas content of the metal phase.
3. A process according to Claims 1 or 2, characterized in that the metal powder is alloyed with elements which enter into compounds with the reaction gas which are present in the materials in the form of finely divided precipitations.
4. A process according to one of Claims 1 to 3 for the making of products from steel powder, characterized in that nitrogen is used as the reaction gas.
5. A process according to Claim 2, characterized in that the process gas is a mixture of argon and nitrogen.
6. An apparatus for the performance of the process according to one of Claims 1 to 5, characterized by a pressure vessel (3) with heating element(s) (4) and a process gas supply pipe (5) and also a capsule (2) filled with metal powder (1) which is disposed in the pressure chamber (7) and to which the reaction gas supply pipe (6) is connected.
EP87110429A 1986-07-21 1987-07-18 Process and apparatus for manufacturing metallic material by hot isostatic compacting of metal powder Expired - Lifetime EP0255005B1 (en)

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AT87110429T ATE56380T1 (en) 1986-07-21 1987-07-18 METHOD AND DEVICE FOR THE MANUFACTURE OF METALLIC MATERIALS BY HOT ISOSTATIC PRESSING OF METAL POWDER.

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DE19863624622 DE3624622A1 (en) 1986-07-21 1986-07-21 METHOD AND DEVICE FOR PRODUCING METALLIC MATERIALS BY HOT ISOSTATIC PRESSING OF METAL POWDER
DE3624622 1986-07-21

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US3743261A (en) * 1971-07-21 1973-07-03 Crucible Inc Furnace and method for heating and compacting powdered metal charges
JPS52141406A (en) * 1976-05-21 1977-11-25 Kobe Steel Ltd Tool steel containing nitrogen made by powder metallurgy
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